AuthorTopic: lens digital equivalents (Read 10003 times)

Imagine a slide projector with an image from a 35mm wide angle film camera on a screen a couple of feet away.

The analogy is the projector is the lens and the screen is the 35mm frame of film. Now take a sheet of ordinary paper and put it against the screen - a bit extreme, but the difference in what is on the paper vs on the screen represents the difference in what's captured using a 1.5 or 1.6 or 1.3 sensor compared to full frame 35mm.

i understand the lens doesnt change, just the image we see. and basically, if i understand correctly, it is because the image reflected (or transmitted, i don't know the correct technical term) by the lens is bigger than the sensor that is receiving the image in the case of say, the 20d. so it's like there is an automatic crop...?!?

Yes, that's it. You have a narrower angle of view with that camera than a regular 135 film camera, which makes it easy to think of as though the image was cropped.

The image isn't cropped, though, you get the same angle of view as in what you see in the viewfinder. Except that the 20D only has 95% viewfinder coverage at 0.9x magnification, that is.

well i thought i understood depth of field.....wishful thinking i guess. what started out as a circle of confusion is fast becoming circles of confusion. i appreciate you guys taking the time to answer questions that to you are probably elementary.amnon

Hello All,I am new to the forum and digital photography.Is there a formula one can use to get a lens' focal length when used on a digital body. I was told to multiply by 1.5 to get "in the ballpark". Any thoughts?Thanks,RBland

i understand the lens doesnt change, just the image we see. and basically, if i understand correctly, it is because the image reflected (or transmitted, i don't know the correct technical term) by the lens is bigger than the sensor that is receiving the image in the case of say, the 20d. so it's like there is an automatic crop...?!? knowing all this, i guess it all really comes into play when you are choosing a wide angle lens. are there any other factors that need to be considered?

lets go back to the reason it LOOKS like you have a longer focal length on a digiSLR.

The cropping reduces the 'field of view' so it's the same as that of a longer focal length lens, by a factor of 1.6 in the case of a D60.

A 50mm lens on a D60 produces images with the same FoV as an 80mm lens on a 1DsMkll. You would get identical images if you cropped the 1DsMkll image from a 50mm lens so it was the same size as the D60 with the same lens. Essentially no difference in any way, except the 1DsMkll would have lower noise at high ISO's (so I believe).

But you don't necessarily get identical images if you use different lenses with the same f stop; ie. 50mm on the D60 at f8 and 80mm on the 1DsMkll at f8. The D60 image will then exhibit greater DoF, at print sizes and sufficient 3-dimensionality of scene where it's possible to discern the DoF differences, of course.

Imagine a slide projector with an image from a 35mm wide angle film camera on a screen a couple of feet away.

The analogy is the projector is the lens and the screen is the 35mm frame of film. Now take a sheet of ordinary paper and put it against the screen - a bit extreme, but the difference in what is on the paper vs on the screen represents the difference in what's captured using a 1.5 or 1.6 or 1.3 sensor compared to full frame 35mm.

abaazov, be careful with the words. The 70-200mm lens is still a 70-200mm lens. When you look in the view finder of your 10D, the image will "appear" the same as the scene viewed with a 112-320mm lens on a regular 35mm camera.

I was thinking of magnification where the 20D and 35mm camera have the same size view finder image.

I know, I was just trying to add on to what you were saying.

Many people talk about 200mm lenses becomming a 320 on some digi SLR and so on but as we know that's not the case. There are many times we might choose to move back from a subect and use a longer lens to shrink the depth of field to get a desired look. Changing from a full size sensor to a smaller one will of course not change the DoF at all.

You are of course quite correct, but as an empirical matter as you say COC is determined in the calculator by a lookup table, which in the DSLR world correlates generally very well with the sensor size, print size and viewing conditions are of course assumed to remain constant for the calculation too.

Perhaps dazzajl does understand what he's talking about, but the way he expressed himself it sounded like he was saying that if you keep 2,3,4 constant and switch from a 20D to a 1DsMkII that the DOF would remain constant, which is not correct. And he also doesn't seem to understand that when you crop an image and enlarge it to give the same print size you do in fact change the DOF.

As you say though, this subject has been done to death on these forums.

That's my last word on the matter, I have no desire to have THE last word here however. (Edit - And besides it's been weeks since we had a DOF thread :p )

"The simple fact is that the size of a piece of film or sensor size has NO bearing on DoF at all."

True. As I suggested, check the link to the DoF calculator and look at the equations for calculating DoF. Format is not a factor.

The mathematical link between format and DoF is through the CoC. The DoF calculations are only useful in relation to a specific format and intended print size. CoC is only meaningful in relation to a print size and a degree of enlargement which is directly related to format size.

Since CoC is a required mathematical input to calculate DoF, then 'format' clearly does have a bearing on DoF. I think it might be described as a 'modifier' of input values.